1. Field of the Invention
The subject invention relates to an ion mobility spectrometer for detecting trace amounts of materials of interest.
2. Description of the Related Art
Ion mobility spectrometry was first reduced to practice in the early to mid 1970""s for the detection and identification of samples carried into an ion mobility spectrometer (IMS) on a stream of clean dry air. Examples of prior art ion mobility spectrometers are shown in U.S. Pat. No. 3,621,240 to Cohen et al., U.S. Pat. No. 3,742,213 to Cohen et al. and U.S. Pat. No. 3,845,301 to Wernlund et al. The IMS described in early literature and patents was capable of distinguishing between ionic species that differed by about ten atomic mass units, but this also depended on molecular shape factors. Further selectivity was achieved by the addition of dopant vapors in the gas stream entering the detector. Typically the dopant would have a charge affinity intermediate that of the target materials and the majority of commonly occurring materials that are of no interest.
The ion trap mobility spectrometer was developed in 1992 and is shown in U.S. Pat. No. 5,200,614 which issued to Jenkins. The ion trap mobility spectrometer allows ion populations longer time in the zero field reaction region of the detector. This facilitates transfer of charge between initially generated ionic species in the detector and the dopant materials. Subsequent charge transfer from the dopant ion to target ions of stronger charge affinity was similarly facilitated in the zero field environment of the detector chamber of the ITMS. U.S. Pat. No. 5,491,337 described the use of ammonia as dopant ion for narcotics detection.
The ITMS has been successfully deployed to detect explosives in the negative ion mode and narcotics in the positive ion mode of operation. It is possible to switch modes of operation by switching the direction of the electric field in the drift region of the detector. A full description of the detector and electrical connection of the ITMS is given in U.S. Pat. No. 5,200,614. Briefly, the ITMS operates by first trapping traces of vapor or particles given off or left behind by explosives and/or narcotics. These trapped samples are vaporized and drawn into the detection system where they are analyzed by a detection system that provides almost 100 times more sensitivity than any prior ion mobility spectrometers. The ITMS ionizes the target vapors and then measures the mobility of the ions in an electric field. The mobility of each target ion differs sufficiently so that each is uniquely identified. This process can take less than three seconds to complete.
Samples are collected on clean paper sample traps either by air sampling with a hand held vacuum sampler or simply wiping suspect surfaces with the trap. Any vapors or microscopic particles of target materials collected on the trap are introduced into the detection system by placing the sample trap in the heated desorption unit shown in FIG. 1. Desorbed vapors are drawn into the ITMS by the action of a small sampling pump. The sampled air leaving the desorption unit is drawn over a semipermeable, elastomeric membrane that allows target vapors to permeate into the detection system. Dust and dirt is excluded by the membrane, thus, protecting the detector from contamination.
The sample molecules that pass through the membrane are carried into the detector in a stream of clean, dry air that is circulated by a small pump (see FIG. 1). The carrier gas with the vaporized sample proceeds through an ionization chamber where both positive and negative ions are formed.
The electric field in the detector""s reaction chamber is at most times zero, but at 20 mS intervals, short pulses are applied across the chamber. This pulsed electric field forces the sample, now in an ionized gas state, to proceed towards the collector electrode. The speed of the ion is related to its size and mass, thus, a measurement of this speed makes substance identification possible. The collector and related electronics pass a constant stream of analogue information from the ITMS into the system computer for digital conversion, analysis and identification.
The ITMS provides high sensitivity due to the increased ionization efficiency compared with standard ion mobility spectrometers. Additionally, detector selectivity is enhanced by the use of the semipermeable membrane in the sampled air stream before the detector. Many organic vapors are transmitted through the membrane and could produce unwanted responses in the detector. These responses are eliminated by the addition of a trace of dopant vapor in the gas stream entering the detector. The dopant is carefully chosen to ensure that it will steal all charge from unwanted ions, and in the absence of narcotics (or explosives in negative ion mode) will produce a single response peak in the spectrum. These are sequentially measured, and produce a positive ion spectrum or plasmagram. Similarly a negative ion spectrum is produced in the negative ion mode for explosives detection.
The time taken to switch modes between positive and negative electric fields in existing equipment is approximately ten seconds. This time is determined by the speed at which the very high voltages employed in the drift region can be discharged and reversed. Unfortunately the residence time of a sample in the detector system is only about five to ten seconds. This is due to the nature of the desorption of particulate samples in the desorber of the product. It is not therefore possible to generate both a positive and negative ion spectrum from the same sample with prior art equipment. The present invention addresses the need to generate positive and negative ion spectra from the same sample and provides a convenient and elegant solution.
Hitherto, there has been little demand for a detector system that would simultaneously detect narcotics and explosives. It would however be helpful in a few applications such as inspection of packages entering the country. The greatest advantage to being able to detect both negative and positive ion spectra from the same sample is to improve both detection capability and selectivity. For example, when providing routine screening of airline passengers and baggage it is important to detect all possible terrorist explosives. Unfortunately there are a few rare explosives that are not very sensitive in the negative ion mode but are more responsive in the positive ion mode. Improved security is achieved by monitoring both positive and negative ion spectra.
In narcotics or positive mode of operation the range of charge affinity that is allowed by the dopant chemistry is greater than is allowed in the negative ion mode. This means that there are more interfering compounds in the narcotics mode than in explosives mode. Unfortunately false positive responses in narcotics mode are procedurally more problematic than explosives false positives. The reverse is true for false negatives. (It would be disastrous to allow a bomb on board an aircraft.) Interdiction forces are already missing 90% of the narcotics entering the country so a few false negatives are not of great concern. Simply put, more selectivity in narcotics detection and more detection capability (sensitivity) in explosives detection is desired. The present invention addresses both these requirements.
Chemical warfare agents are either strongly electropositive or strongly electronegative. Any IMS system designed for the full range of chemical weapons threat must be able to detect both positive and negative ion spectra simultaneously. The present invention would be particularly applicable to chemical warfare agent detection.
The invention is directed to an improved ion trap mobility spectrometer and a method for testing for the presence of at least one substance of interest in a sample of air. The spectrometer includes a desorber for receiving a sample trap that has been placed in communication with a potential source of substances of interest. A pump is provided for directing a flow of air across the sample trap for delivering substances on the sample trap from the desorber to an ionization chamber. A drift chamber is disposed adjacent the ionization chamber and a collector electrode is disposed at an end of the drift chamber remote from the ionization chamber. A plurality of sequentially spaced grid electrodes are disposed in the drift chamber between the ionization chamber and the collector electrode.
The ionization chamber functions to bombard molecules in the sample gas to produce ionized molecules. At most times, the electric field in the ionization chamber is zero. However, short pulses are applied across the chamber to propel the ionized gas from the ionization chamber into the drift chamber. The grid electrodes in the drift chamber are operated at a first polarity for a first selected period of time to cause at least a first species of molecules to be directed toward the collector electrode. The collector electrode is connected to a signal processor and a display means. The signal processor identifies at least the first species of molecules impinging thereon. A display means then produces at least one plasmagram for identifying at least certain species of ions, as collected on the collector electrode and analyzed by the signal processor.
The ion trap mobility spectrometer further includes switching means for rapidly reversing polarity of the grid electrodes. The reverse of polarity may propel other species of ions toward the collector electrode. In this manner, a single sample of air drawn from a single sample trap can be analyzed for two different species of substances of interest or can be analyzed to assess more accurately the presence of certain species of substances of interest that may have other molecules that can be detected better in a positive mode as well as molecules that can be detected in a negative mode.